The purpose of this study is to assess the effect of ampicillin on neonatal colonization and infecton with group B streptococci (GBS) when administered to women in labor who are colonized with GBS and have major risk factors associated with neonatal infection. The risk factors are premature labor (less than or equal to 36 weeks of gestation) and prolonged rupture of the amniotic membranes (greater than or equal to 12 hours duration). Screening for GBS will be done by vaginal and rectal swabs obtained during the second trimester and confirmed by vaginal culture during labor. The predictive value and quantitative effect of prenatal vaginal and rectal cultures with respect to vaginal colonization in labor and neontal colonization will be tested. The interval between prenatal cultures and delivery, the use of antibiotics during this interval, and the correlation between serotypes isolated during prenatal visits and those isolated at delivery will also be examined. The frequency of GBS colonization at parturition in women whose prenatal cultures were negative will be tested. We will monitor the side effects of ampicillin on mother and newborn infant. The therapeutic strategy is not expected to eliminate all cases of GBS early-onset disease. It is expected to prevent most cases since the great majority of infections are associated with risk factors. Furthermore, mortality is generally limited to this group. Full-term infants without other risk factors who develop early-onset-GBS disease are usually treated successfully. By limiting the administration of ampicillin to selected women in labor, we decrease the risk of ampicillin reactions in the obstetrical population.